Time delay fuse

ABSTRACT

A time delay fuse including a fuse casing, terminals located on the exterior of the casing, a hollow electrically insulated inner core having first and second holes in the walls thereof, a fusible element that makes electrical connection to the terminals and is disposed on the surface of the core such that the length of the element is greater than the distance between the terminals, the element passing through the holes, and a material that is deposited on a portion of the element passing through the interior of the core and lowers the melting temperature of that portion.

BACKGROUND OF THE INVENTION

The invention relates to time delay fuses.

Time delay fuses are used in connection with equipment having temporarycurrent surges, such as motors and transformers. Time delay fuses oftenemploy a fusible element and a spring-loaded heat mass. A deposit ofsolder retains the heat mass from movement by the spring. The dimensionsof the fusible element are selected such that it melts quickly undershort-circuit conditions (e.g., 30 times the rated current of the fuse).However, when lower overload conditions (e.g., 2 to 4 times ratedcurrent) persist for a predetermined amount of time, the solder insteadmelts, releasing the heat-mass to break the circuit.

Another approach used in time delay fuses utilizes the "M-effect," whichis achieved by depositing a tin-bearing metal on the surface of acopper, silver, brass, or phosphor-bronze element such that the twometals alloy. The resulting alloy has a lower melting point than theelement material alone. At low-overload conditions, the fusible elementslowly generates heat. Eventually, the temperature rise is sufficient tomelt the alloy region at the solder/tin deposit and thereby break thecircuit. The time needed to generate the necessary heat results in adelay.

SUMMARY OF THE INVENTION

our invention features, in general, a time delay fuse the interior ofwhich contains a hollow insulated core. A fusible element is connectedat each end to a fuse terminal and is disposed on the surface of thecore such that the total length of the fusible element exceeds thedistance between the two terminals. A portion of the fusible element iswithin the core, and a material deposited on this portion lowers themelting temperature at this portion of the element, providing delayedmelting at this portion of the fusible element and breaking of thecircuit at low overloads.

This time delay fuse design eliminates the need for a heat-mass andspring assembly, reducing both manufacturing cost and packaging size.Additionally, a fuse according to the present invention can have both alow nominal current rating, and a high transient in-rush current rating(i.e., it can withstand short periods of very high overload conditions).The in-rush current rating of a fuse is determined by thecross-sectional area of the fusible element. However, increasingcross-sectional area to increase the in-rush current rating decreasesthe resistance per unit length of the fusible element, requiring that alonger length be used to obtain the resistance needed for the nominalcurrent rating. Because the fusible element in the present invention islonger than the distance between the two terminals, it is possible toinclude in a small package a long fusible element with a largecross-sectional area to increase the in-rush rating while maintainingthe desired nominal rating. In one exemplary embodiment, this isaccomplished by spiral-winding the fusible element around the surface ofthe core.

In preferred embodiments, the portion of the fusible element carryingthe melting temperature lowering material extends between two holes inthe core. The fuse casing is cylindrical and has an inner diameter thatis less than the sum of the exterior diameter of the core and eighttimes (most preferably three times) the thickness of the fusibleelement. This provides a relatively small volume for the interrupt arcand metal vapor resulting when a high overload current is applied. Thehigh pressures thus developed are sufficient to quench the arc, thusboth stopping all current flow through the fuse and preventing the fusefrom exploding.

Other advantages and features of the invention will be apparent from thefollowing description of a preferred embodiment thereof and from theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a time delay fuse according tothe invention;

FIG. 2 is a plan view of a subassembly of the FIG. 1 fuse; and

FIG. 3 is a vertical sectional view, taken at 3--3 of FIG. 2, showingthe orientation of the fusible element of the FIG. 2 subassembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown time delay fuse 10, which includestubular fuse casing 12 (made of glass melamine glass), on which arecrimped end ferrules 14 and 16. The length of fuse casing 12 is 1.290",and the outer and inner diameters are 0.352"+0.005" and0.250"+0.005"-0.000", respectively. Within casing 12 is a hollow innercore 18 (also made of glass melamine glass), around which isspiral-wound fusible element 20 made from wire (phosphor-bronze alloyC524000). The length of inner core 18 is 1.275", and the outer and innerdiameters are 0.227"+0.000"-0.005", and 0.187"+0.005", respectively.

The size of fusible element 20 determines the ampere rating and timedelay characteristics of fuse 10. For example, in a fuse with a currentrating of 3/16 amperes, fusible element 20 has a diameter of 0.0030" andis 32" to 38" long; this fuse will tolerate a 10 millisecond currentpulse of up to 75/16 amperes (25×3/16). In a fuse with a current ratingof 1.8 amperes, fusible element 20 has a diameter of 0.0075" and is 2.0"long; the fuse will tolerate a 10 millisecond current pulse of up to 45amperes (25×1.8). The ends of fusible element 20 electrically contactend ferrules 14 and 16.

Referring to FIGS. 2 and 3, fusible element 20 passes through holes 22and 24 in the wall of inner core 18. Holes 22 and 24 are located at theaxial midpoint of inner core 18, and are offset 180° from one another.Tin bead 26 is deposited at the midpoint of that portion of fusibleelement 20 that passes through the interior of inner core 18.

In manufacture, tin bead 26 is first deposited on fusible element 20,which is then passed through holes 22 and 24 until tin bead 26 iscentered with respect to inner core 18. Exact positioning of tin bead 26is not critical, but the bead must not be allowed to contact the innersurface of core 18. Fusible element 20 is then spiral-wound around theouter surface of inner core 18 to result in assembly 32 (see FIGS. 2 and3), the total number of coils being determined by the length of fusibleelement 20. For a 3/16 amp fuse of 0.003" wire, element 20 isapproximately 32" to 38". Adjacent coils must not touch, and should beequally spaced along the length of inner core 18.

After spiral-wrapping fusible element 20, the two free ends of element20 are trimmed and tucked over the ends of inner core 18 and into thecore's hollow interior. Eyelets 28 and 30 (see FIG. 1) are then insertedinto the ends of inner core 18 to secure and provide electrical contactwith the ends of fusible element 20. The resulting inner coresubassembly is then inserted into fuse casing 12, and end ferrules 14and 16 are installed, with solder (not shown) located between ferrules14 and 16 and eyelets 28 and 30. Ferrules 14 and 16 are then crimped tothe fuse casing 12, and the ends are subjected to induction-heating tomelt the solder, electrically connecting each ferrule 14 and 16 to itsassociated eyelet 28 and 30.

In operation, when the current passing through fuse 10 remains attwenty-five times the rated nominal current of the fuse for longer than10 milliseconds, fusible element 20 ionizes and forms an interrupt arc.At higher currents, element 20 ionizes sooner. Because fusible element20 is largely confined in the relatively small volume defined by theregion between the inner surface of fuse casing 12 and the outer surfaceof the inner core 18, high pressures develop in this inter-tubularregion during ionization. These pressures quench the interrupt arc, thusboth stopping all current flow through the fuse and preventing the fusefrom exploding.

At low overload currents, for example two times the rated current,fusible element 20 is such that it will not ionize. Rather, the portionsof element 18 supported by core 18 will conduct heat to core 18, and theportion in the interior of core 18 will rise in temperature and have thehottest temperature. When the tin bead region of fusible element 20reaches its melting temperature, the region fuses, breaking electricalcontact between end ferrules 14 and 16.

Other embodiments of the invention are within the scope of the followingclaims.

What is claimed is:
 1. A time delay fuse comprising: an elongated fusecasing defining an interior chamber and having a longitudinal axis,firstand second terminals located along said axis on the exterior of saidcasing at opposite ends of said casing, a hollow, elongated,electrically non-conductive core located within said interior chamber,said core extending along said longitudinal axis and havingapproximately the same shape as said interior chamber, said core havingan inner region therein, said fuse having an outer region around saidcore and inside of said fuse casing all of the way around said core, afusible element making electrical connection at one end to said firstterminal and electrical connection at the other end to said secondterminal, said fusible element being disposed on the surface of saidinsulated core in said outer region such that the length of said elementis greater than the distance between said first and second terminals,said fusible element having a portion disposed within said inner regionwithin said hollow core, and a material deposited on said portion ofsaid fusible element within said inner region within said core forlowering the melting temperature of that portion, said material notcontacting and being spaced from said non-conductive core.
 2. The timedelay fuse of claim 1 wherein said fusible element is spiral-wound onthe surface of said core.
 3. The time delay fuse of claim 1 wherein saidfusible element is a wire.
 4. The time delay fuse of claim 3 whereinsaid wire is a phosphor-bronze wire.
 5. The time delay fuse of claim 1wherein said material is tin.
 6. The time delay fuse of claim 1 whereinsaid hollow core has first and second holes at respective first andsecond locations in the wall of said core, and said portion of saidfusible element within said core extends between said first and secondholes.
 7. The time delay fuse of claim 6 wherein said first, and saidsecond locations are generally at the axial midpoint of said core. 8.The time delay fuse of claim 1 wherein said core is a cylindrical body,and said interior chamber is cylindrical.
 9. The time delay fuse ofclaim 8 wherein the diameter of said interior chamber is less than eighttimes the thickness of said fusible element plus the exterior diameterof said core.
 10. The time delay fuse of claim 9 wherein the diameter ofsaid interior chamber is less than eight times the thickness of saidfusible element plus the exterior diameter of said core.
 11. The timedelay fuse of claim 1, further including an arc-quenching materialsurrounding a portion of said element that is disposed on said core. 12.The time delay fuse of claim 11 wherein said arc-quenching material isquartz crystal.
 13. The time delay fuse of claim 1 wherein said firstand second terminals are end cap terminals.